Silicon is commonly used as a substrate material for infrared reflectors and windows in 1.5~8um. It is frequently used for laser mirrors because of its high thermal conductivity and low density. Silicon lenses are popular for many applications in IR. They are typically used for imaging applications in the 3~5um region of the spectrum, making them excellent for security and military applications. They can also be used to collimate infrared lasers. 
These lenses are coated with a BBAR coating that provides 98% transmission in the 3~5um spectral range.